If I know there is going to be a triple sink, I prefer to install three separate connections into the wall. Failing that, a connection at each sink with the disposer connected to one of them is the next best option. If you only have one drain opening which I assume to be your case, then getting the second sink connected past the disposer can be a challenge. How you do it will be specific to your particular installation, so we could give specifics without knowing where the drain is in relation to the three bowls, then height out of the wall, even the size of the disposer since some, such as G.E. are quite small, etc..